A host resistance gene has been identified in mice which specifically suppresses in vitro and in vivo replication of the MCF class of murine leukemia viruses. Designated Rmcf, the gene is dominant for resistance, is located on chromosome 5 very close to the locus for hammer-toe (Hm), and is carried by DBA/1, DBA/2 and certain sublines of CBA (i.e., those related to CBA/H). The biological importance of this virus resistance gene is being explored in pathogenesis experiments, including tests of mouse strains partially congenic for Rmcf(r).